First you need to be sure you understand the basis of the grade. Recheck the syllabus to see how the grades are determined.
If your paper is supposed to be ten pages long and you turn in seven pages, double space in fourteen-point type, you’re not going to get an A. If it’s a research paper, then citing Wikipedia does not count. Good grades are about not only doing the work, but doing it well. It’s about quality—your work should be grammatical, logical, aligned with instructions, and thoughtful, as a bare minimum.
So if you’re struggling to understand the lecture, doing badly on quizzes, or seeing lots of red marks on your midterm paper, then you need to go see your professor to ask what you’re doing wrong and how to improve. In grading in non-quantitative courses, a subjective factor often exists, and the sincerity of your effort could be the difference between a B and a B+. High grades are not an entitlement because you pay the bills and deign to attend class—they are a reflection of real effort and energy.
If you’ve been to the professor, however, and she will not budge or cannot be found (which sometimes happens with part-time faculty), then go to the department chair, the boss of the department in which the instructor teaches. This is the person who can locate a wayward professor, seek evidence of why a grade was given, and help negotiate an understanding and an amicable resolution. Do not try to go above this person; you’ll get marked as a troublemaker if you write to the president of the college and copy the board of trustees about your grade—this approach does not make friends. Furthermore, these officials are not empowered to change grades; that is the purview of the faculty.
With a disputed grade, you may be in a good position to make your case, if you’ve kept copies of emails and all assignments, graded papers, and exams. To prove that you turned in a paper on a specific date, for example, you may need a computer record. Keep everything until you have your diploma in hand.